Biomass (ecology)

Apart from bacteria, the total global live biomass has been estimated as 560 billion tonnes C,[1] most of which is found in forests.[2]

Shallow aquatic environments, such as wetlands, estuaries and coral reefs, can be as productive as forests, generating similar amounts of new biomass each year on a given area.[3]

Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms, plants or animals.[4] The mass can be expressed as the average mass per unit area, or as the total mass in the community.

How biomass is measured depends on why it is being measured. Sometimes, the biomass is regarded as the natural mass of organisms in situ, just as they are. For example, in a salmon fishery, the salmon biomass might be regarded as the total wet weight the salmon would have if they were taken out of the water. In other contexts, biomass can be measured in terms of the dried organic mass, so perhaps only 30% of the actual weight might count, the rest being water. For other purposes, only biological tissues count, and teeth, bones and shells are excluded. In some applications, biomass is measured as the mass of organically bound carbon (C) that is present.

Apart from bacteria, the total live biomass on Earth is about 560 billion tonnes C,[1] and the total annual primary production of biomass is just over 100 billion tonnes C/yr.[5] The total live biomass of bacteria may be as much as that of plants and animals[6] or may be much less.[7] The total amount of DNAbase pairs on Earth, as a possible approximation of global biodiversity, is estimated at 5.0 x 1037, and weighs 50 billion tonnes.[8] In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).[9]

The bottom of the pyramid represents the primary producers (autotrophs). The primary producers take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates. This mechanism is called primary production. The pyramid then proceeds through the various trophic levels to the apex predators at the top.

When energy is transferred from one trophic level to the next, typically only ten percent is used to build new biomass. The remaining ninety percent goes to metabolic processes or is dissipated as heat. This energy loss means that productivity pyramids are never inverted, and generally limits food chains to about six levels. However, in oceans, biomass pyramids can be wholly or partially inverted, with more biomass at higher levels.

Terrestrial biomass generally decreases markedly at each higher trophic level (plants, herbivores, carnivores). Examples of terrestrial producers are grasses, trees and shrubs. These have a much higher biomass than the animals that consume them, such as deer, zebras and insects. The level with the least biomass are the highest predators in the food chain, such as foxes and eagles.

In a temperate grassland, grasses and other plants are the primary producers at the bottom of the pyramid. Then come the primary consumers, such as grasshoppers, voles and bison, followed by the secondary consumers, shrews, hawks and small cats. Finally the tertiary consumers, large cats and wolves. The biomass pyramid decreases markedly at each higher level.

In turn, small zooplankton are consumed by both larger predatory zooplankters, such as krill, and by forage fish, which are small, schooling, filter-feeding fish. This makes up the third level in the food chain.

Apex predators, such as orcas, which can consume seals, and shortfin mako sharks, which can consume swordfish, make up the fifth trophic level. Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels.

Marine environments can have inverted biomass pyramids. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers grow and reproduce slowly.

There is an exception with cyanobacteria. Marine cyanobacteria are the smallest known photosynthetic organisms; the smallest of all, Prochlorococcus, is just 0.5 to 0.8 micrometres across.[10] Prochlorococcus is possibly the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be several octillion (~1027) individuals.[11]Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans.[12] The bacterium accounts for an estimated 20% of the oxygen in the Earth's atmosphere, and forms part of the base of the ocean food chain.[13]

There are typically 50 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. In a much-cited study from 1998[6] the world bacterial biomass was calculated to be 350 to 550 billions of tonnes of carbon, equal to between 60% and 100% of the carbon in plants. More recent studies of seafloor microbes have cast considerable doubt on that, one study in 2012[7] reduced the calculated microbial biomass on the seafloor from the original 303 billions of tonnes of C to just 4.1 billions of tonnes of C, reducing the global biomass of prokaryotes to 50 to 250 billions of tonnes of C. Further, if the average per cell biomass of prokaryotes is reduced from 86 to 14 femtograms C[7] then the global biomass of prokaryotes is reduced to 13 to 44.5 billions of tonnes of C, equal to between 2.4% and 8.1% of the carbon in plants.

Estimates for the global biomass of species and higher level groups are not always consistent across the literature. Apart from bacteria, the total global biomass has been estimated at about 560 billion tonnes C.[1] Most of this biomass is found on land, with only 5 to 10 billion tonnes C found in the oceans.[1] On land, there is about 1,000 times more plant biomass (phytomass) than animal biomass (zoomass). About 18% of this plant biomass is eaten by the land animals.[15] However, in the ocean, the animal biomass is nearly 30 times larger than the plant biomass.[16] Most ocean plant biomass is eaten by the ocean animals.[15]

Humans comprise about 100 million tonnes of the Earth's dry biomass,[31]domesticated animals about 700 million tonnes, and crops about 2 billion tonnes.[citation needed] The most successful animal species, in terms of biomass, may well be Antarctic krill, Euphausia superba, with a fresh biomass approaching 500 million tonnes,[28][32][33] although domestic cattle may also reach these immense figures.[citation needed] However, as a group, the small aquatic crustaceans called copepods may form the largest animal biomass on earth.[34] A 2009 paper in Science estimates, for the first time, the total world fish biomass as somewhere between 0.8 and 2.0 billion tonnes.[35][36] It has been estimated that about 1% of the global biomass is due to phytoplankton,[37] and fully 25% is due to fungi.[38][39]

Grasses, trees and shrubs have a much higher biomass than the animals that consume them

Globally, terrestrial and oceanic habitats produce a similar amount of new biomass each year (56.4 billion tonnes C terrestrial and 48.5 billion tonnes C oceanic).

Net primary production is the rate at which new biomass is generated, mainly due to photosynthesis. Global primary production can be estimated from satellite observations. Satellites scan the normalised difference vegetation index (NDVI) over terrestrial habitats, and scan sea-surface chlorophyll levels over oceans. This results in 56.4 billion tonnesC/yr (53.8%), for terrestrial primary production, and 48.5 billion tonnes C/yr for oceanic primary production.[5] Thus, the total photoautotrophic primary production for the Earth is about 104.9 billion tonnes C/yr. This translates to about 426 gC/m²/yr for land production (excluding areas with permanent ice cover), and 140 gC/m²/yr for the oceans.

However, there is a much more significant difference in standing stocks—while accounting for almost half of total annual production, oceanic autotrophs account for only about 0.2% of the total biomass. Autotrophs may have the highest global proportion of biomass, but they are closely rivaled or surpassed by microbes.[40][41]